UVCS/EIT composite image. The Sun's outer atmosphere a
s it appears in ultraviolet light emitted by electrically charged oxygen flowing
away from the Sun to form the solar wind (region outside black circle), and the
disk of the Sun in light emitted by electrically charged iron at temperatures n
ear two million degrees Celsius (region inside circle). This composite image tak
en by two instruments (UVCS, outer region and EIT, inner region) shows dark area
s called coronal holes at the poles and across the disk of the Sun where the hig
hest speed solar wind originates. UVCS has discovered that the oxygen atoms flow
ing out of these regions have extremely high energies corresponding to temperatu
res of over 200 million degrees Celsius and accelerate to supersonic outflow vel
ocities within 1.5 solar radii of the solar surface. The structure of the corona
is controlled by the Sun's magnetic field which forms the bright active regions
and the ray-like structures originating in the coronal holes. The composite ima
ge allows one to trace these structures from the base of the corona to millions
above the solar surface.